In the development of alternative drive concepts for automotive applications electrical propulsion in conjunction with a low temperature fuel cell system as an electrochemical energy converter has particularly increased in significance. The choice of the fuel has in this respect a substantial influence on the complexity of the fuel cell system. When using organic fuels such as for example methanol or gasoline, the system complexity increases because of the required reformation of fuel to form hydrogen-rich gas. The use of pure hydrogen results in a substantial simplification of the system.
In the following hydrogen will principally be referred to as the gaseous fuel, the invention is however not restricted to the use of hydrogen as the gaseous fuel but rather relates in principle to the supply of any desired gaseous fuels to any desired consumers which have a changing requirement for fuel in operation.
For the more detailed explanation of the invention a fuel cell system will be considered which is supplied with compressed hydrogen from a hydrogen tank. The hydrogen gas is supplied in dependence on the load to the fuel cell stack at the anode side. Oxygen or impure oxygen (air) is supplied to the cathode side of the fuel cell stack in dependence on the load. The desired load value results essentially from the performance demanded of the overall vehicle as selected by the driver (acceleration, braking etc.) and also on the power requirement of the electrical loads which are involved in the fuel cell system.
With a fuel cell system of this kind it is necessary to restrict the pressure of the hydrogen gas from a high pressure level in the tank to a lower pressure level for the fuel cell system at the outlet of the tank. The low pressure level is not a single absolute pressure point but rather a range of pressures at low level.
The known delivery systems for delivering a gaseous fuel have been designed hitherto so that at least one mechanically operating pressure regulating valve and also a control valve are provided connected in series between the source of the gaseous fuel with a higher pressure level and the fuel cell system. The task of the mechanically operating pressure valve has hitherto been to produce a constant feed pressure and the control valve is controlled in accordance with the load demands in order to control the mass flow of fuel supplied to the fuel cell stack.
The control valve had to be designed to handle the whole pressure range between the constant feed pressure of the mechanical pressure regulating valve and the smallest pressure prevailing at the fuel cell side of the valve. This leads to relatively high demands being placed on the quality of the control valve and its ability to respond.